To prevent the crystallization of water entering into the concrete solution, it is necessary to maintain a certain temperature of the filled mass. The point is that the cement( cement) reacts with the liquid, not with the ice. And since the final hardening of concrete takes place for a long time( up to 4 - 5 weeks, depending on the characteristics of the work and composition of the mixture), then its heat treatment is carried out constantly, to the full readiness of the structure being constructed.
It is clear that warm-up is necessary only in the cold season. This allows you to work in any season, regardless of the ambient temperature. There are many methods, but, perhaps, the most common is the heating of the concrete mixture with electrodes. Such conductors of electric current differ in shape, size and specificity of placement.
But the technology and the principle of their action remains unchanged - the concrete is heated by the electric field, which is formed between the electrodes when the voltage is applied to them. The solution becomes an element of a current-carrying chain( with its internal resistance), in which the energy of the electrical is transformed into a thermal one. By adjusting the voltage rating, you can achieve the required warm-up temperature. Depending on the features of the "processed" design, the optimal variant of these elements is selected.
Kinds of electrodes
Rod
As such, reinforcing bars are usually used, although narrow metal strips can be installed( composite reinforcement, of course, will not work, but for reinforcement - that's what it takes).Its length should be slightly thicker than the fill( for inclusion in the chain), and the cross-section is selected based on its design features and the layout of the electrodes( usually for private housing construction not more than 10 mm).To make the reinforcement easier to enter the solution, one end of it is sharpened.
Rod electrodes allow you to warm up the "fill" with the configuration of any complexity and shape, so they are used most often, especially for individual construction. They are located perpendicular to the longitudinal axis of the structure. And so that they do not come into contact with the bars of the reinforcing cage.
String
In fact, this is a kind of the same rod, but the arrangement is along the axis of the formwork. They are used for heating structures with a small cross-section and a long length( beams, columns and a number of others).To simplify the connection of wires, the edges protruding from the formwork are bent upwards( the letter "G").
Banded
Are pieces of iron strips( 20 - 50 mm, thickness 3), which are laid on top of the filled solution. Such heating is used to fill a small thickness( massive screed, slab and the like), while all the elements are placed on one side of the structure.
Plate
Located on opposite sides of the pouring, on the inside of the formwork. Their dimensions are selected in accordance with its parameters. Naturally, they are installed in pairs, the number of which and the arrangement are determined individually for each design.
Heating types
Through-hole( internal, submersible)
Suitable for structures with a large thickness or complex shape. From the name it is clear that the electrodes are placed inside the grouted mass of the solution. The general rule is that the electrodes are installed at a distance of at least 3 cm from the formwork element.
Peripheral( surface, sewn)
Under the stripes, a lining is installed. In practice, pieces of roofing material are often used for this, which allows such electrodes to be easily removed and used repeatedly.
General rule
If a metal frame is installed in the formwork, the uses a voltage of more than 127 V IS FORBIDDEN.For non-reinforced structures it can be no more than 380 V.
What to consider when heating concrete
- As the solidified material solidifies, its electrical resistance changes, as moisture evaporates. Therefore, it is necessary to systematically correct the power of the supplied current, therefore an adjustment element( for example, a rheostat, a transformer with several outputs) must be included in the circuit.
- The surface of the structure to be heated must be covered with materials that reduce heat loss. This can be sawdust, mats, foil, roofing paper and the like. Otherwise, the heating process itself becomes meaningless.
- With the rod method, it is necessary to observe the same distances between the electrodes in one row as well as in the neighboring rows. This will ensure the uniformity of loading the "lines" and eliminate the skew of the phases.
- Reduction of energy costs can be achieved by introducing into the solution specific additives, plasticizers, accelerating the process of solidification of concrete.
- Specialists do not recommend the use of electrode heating for small structures. There are other methods for this.
- As a "power supply" you can not use a direct current source, since in this case electrolysis of the liquid can not be avoided.
- With small fill volumes, welding transformers can be used as a voltage source.
- There is no single recommendation for placing electrodes on( in) filling the solution. The scheme is determined individually and depends on external conditions, form parameters, cement grade and a number of other factors.
- At certain time intervals( depending on the specifics of the work), the temperature is measured. For this, special "pits" are made.
- IS FORBIDDEN.When using reinforcing bar rods as electrodes to work with a voltage of more than 60 V. In exceptional cases( more than this denomination) - only with additional measures and locally( on certain segments of the design).
For the production of a quality artificial stone from the solution, it is recommended to heat the mass in a complex manner, combining several techniques, including "passive"( "thermos").